1. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw 12. The interstellar medium: gas 12.3 H I clouds (and IS absorption lines) 12.4 Dense molecular clouds 12.5 Interstellar masers 12.6 Note on pressures in IS gas NGC1232

2. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw H I clouds and interstellar (IS) absorption lines Wide distribution throughout galactic disk to R ~ 20 kpc Greatest density of clouds for 4 kpc < R < 14 kpc Number along a given lines of sight in glactic plane ~ 7 or 8/kpc Typical size a few pc to a few tens of parsecs Typical mass M HI ~ 100 M ⊙ Temperature T ~ 90 K Radio emission λ = 21 cm, frequency f = 1420.406 MHz

3. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw 21-cm emission in other spiral galaxies This image shows the HI emission in the face-on spiral M101 using the Westerbork radio telescope in Holland. The HI distribution is easier to determine than in the Milky Way, because we can observe this galaxy from an external vantage point.

4. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw HI and CO distribution with radius R in the Galaxy

5. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw The Magellanic Stream and H I high velocity clouds represent weak sources of 21-cm emission located well away from the galactic plane. They are the result of tidal interation of the Galaxy on the satellite galaxies, the Magellanic Clouds.

6. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw The 21-cm line The 21-cm line of neutral atomic hydrogen is known as a hyperfine structure transition. Metastable upper energy state has e and p spins parallel, lower energy state, antiparallel.

7. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Lifetime of uper energy state ~ 11 million years, with spontaneous emission of a photon. The upper energy state is populated by collisions which are relatively frequent (one such excitation per H atom occurs about every 400 yr). Three quarters of all H atoms are on average in the upper state.

8. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw H I line formation

9. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw IS absorption lines star in galactic plane H I cloud observer on Earth Spectrum of a distant galactic plane star contains narrow IS absorption lines produced by heavy elements in IS gas clouds.

10. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw IS lines in a stellar spectrum

11. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw IS lines due to Na, Ca, Ti, K, Fe and molecules CN, CH, CH + are known in optical region IS lines due to C, N, O, Mg, Si, P, S, Cl, Ar, Mn, Fe and molecules H 2, HD, CO are observed in the ultraviolet

12. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Above: narrow IS absorption lines in the spectrum of a distant galactic plane star differ markedly from the broader stellar line. Right: multiple components in the IS NaD line due to clouds at different velocities in the line of sight.

13. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw IS line strengths give information on chemical composition of IS H I clouds. Some heavy elements (e.g. Ca) are greatly depleted in IS clouds (deficient by a factor ~ 2 × 10 -4 ), while others (e.g. C, N, O) are hardly changed relative to solar composition. Element depletion is by heavy element accretion onto dust grains, thereby removing some refractory elements from the gas.

14. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw The depletion of heavy elements in H I clouds as deduced by the strengths of IS absorption lines. There is no correlation of depletion factor with atomic weight A, but a good correlation with the element’s condensation temperature T c.

15. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Dense molecular clouds The most common molecules are H 2, CO, CN, OH, H 2 CO. Most molecules (but not H 2 ) give characteristic radio emission lines, which allow them to be identified. Over 50 have been detected. Absorption lines are usually seen for OH, always for H 2 CO. Molecule formula discovery λ number sources hydroxyl OH 1963 1.8 cm ~600 ammonia NH 3 1968 1.3 cm 12 water H 2 O 1968 1.3 cm 35 formaldehyde H 2 CO 1969 6.2 cm ~150 carbon monoxide CO 1970 2.6 mm 60

16. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Some interstellar molecules observed in the IS medium. The first 5 are found as optical/UV IS absorption lines in stellar spectra; the second set are seen as radio emission lines in dense molecular clouds, (or as radio absorption lines when distant sources are seen through dense molecular clouds).

17. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Microwave spectrum of emission lines from a dense molecular cloud

18. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Properties of dense molecular clouds Temperature T ~ 10 to 30 K Number densities n ~ 10 8 – 10 12 molecules m -3 ; mass density ρ ~ 10 -15 kg.m -3 Cloud mass may be ~ 10 3 M ⊙ Cloud size ~ 10 pc Dense molecular clouds are often very dusty

19. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Note that dust shields molecules from UV radiation from stars, which would dissociate most molecules. Also dust surfaces provide a site for the formation of the H 2 molecule. Other molecules can form from gas phase reactions.

20. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Dense molecular clouds are under gravitational collapse because there is enough mass for self gravity to pull them together. They are consequently sites of star formation

21. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Above: galactic distribution of CO in molecular clouds Below: CO cloud radial velocity vs galactic longitude

22. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Some H II nebulae which are also associated with dense molecular clouds 1.η Car 2. M20 Trifid nebula 3. Orion nebula 4. M16 Eagle nebula

23. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Note on pressures in the IS gas Phase n (m -3 ) T (K) P (Pa) H II 10 8 9000 10 -11 H I 10 7 90 10 -14 dense molecular 10 9 to 10 12 10 – 30 10 -13 – 10 -10 hot H I 3 × 10 5 5000 10 -14 coronal gas 10 3 10 6 10 -14 P = n k T P pressure (Pa); n number density (m -3 ); T absolute temp. (K); k Boltzmann’s constant (J.K -1 )

24. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw H I clouds are in pressure equilibrium with the hot H I and coronal gas intercloud medium The pressure of H II clouds is much higher than the surrounding medium (normally H I ) and they therefore expand supersonically (~ 10 km/s) into the surrounding gas.

25. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Dense molecular clouds also have much higher pressures, but this is the result of their high masses, causing them to collapse and be compressed under their self gravity (they are the only phase of the ISM where self-gravity dominates over gas pressure) Note IS gas pressures are always very low. On Earth 1 atmosphere ≈ 10 5 Pa, much higher than in ISM

26. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Interstellar masers MASER: Microwave Amplification by Stimulated Emission of Radiation Observed in OH lines (λ ~ 18 cm) and sometimes in lines of H 2 O (1.35 mm) and SiO (6.95 mm, 3.47 mm) IR pumping from thermal IR from dust can cause a population inversion of OH in gas in a metastable upper level – this is a condition for maser action.

27. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw Stimulated emission can occur, resulting in a very intense emission line from a small region of space (generally a few tens of A.U. across). Maser sources are compact and probably occur in dusty regions associated with star formation or in circumstellar dust shells around M-type stars. There are several OH and H 2 O maser sources in the dense molecular cloud associated with the Orion nebula – possibly where new-born stars are still enshrouded in a coccoon of circumstellar dust grains.

28. ASTR112 The Galaxy Lecture 9 Prof. John Hearnshaw END OF LECTURE 9